Method for determining relationship between difference of object lens location and driving control effort in optical storage device

ABSTRACT

A method is used in an optical storage device having a pickup head. The pickup head includes an object lens, a first light source and a second light source. The object lens focuses the laser light generated by the first light source onto a first focal point or focuses the laser light generated by the second light source onto a second focusing point. A driving control effort is used to change the location of the object lens. The method includes the following steps: providing the driving control effort to move the object lens; measuring a first parameter when a signal reaches a first state; measuring a second parameter when the signal reaches a second state; and determining the relationship between the difference of object lens location and the driving control effort according to a predetermined distance, the first parameter, and the second parameter.

CROSS REFERENCE TO RELATED APPLICATIONS

This divisional application claims the benefit of co-pending U.S. application Ser. No. 10/904,872, which was filed on Dec. 1, 2004 and is incorporated herein by reference.

BACKGROUND

The present invention relates to an optical storage device, and more particularly, to a method for determining a relationship between difference of object lens location and a driving control effort in an optical storage device.

A personal computer (PC) or other electronic system often includes an optical storage device, which has become a widely applied storage medium. FIG. 1 shows a block diagram of a conventional optical storage device 100. In FIG. 1, the optical storage device 100 comprises a spindle motor 110, an optical disc 120, a pickup head 130, a signal processing unit 140, a focusing controller 150, a micro controller unit (MCU) 160, and a power driver 170. The optical disc 120 is of CD or DVD type, which contains at least two layers: a surface layer and a reflective layer. The surface layer is defined as the disc surface at the side towards to the pickup head. The reflective layer is defined as the junction layer between the protective material and metal surface on which the information pits is located. Normally, there is one DVD laser diode (LD) and one CD laser diode installed on the pickup head 130. An object lens is used to focus the laser light generate by the DVD laser diode or the CD laser diode onto the optical disc 120, then the pickup head 130 can reproduce the data stored on the optical disc 120 according to received reflected light. A focusing actuator, also called a focusing coil, of the pickup head moves the object lens perpendicular to the surface of the optical disc 120 according to a driving control effort (in this example, the driving control effort is a driving voltage) generated by the power driver 170, hence the location of a focal point of the laser light generated by one of the LDs can be controlled.

When controlling the focal point of the laser light to jump between different layers of the optical disc 120, or processing an optical disc identification procedure, the driving voltage used to drive the focusing actuator would continuously be altered. By altering the driving voltage, the object lens can be moved, hence the location of focal point of the laser light can be controlled. The relationship between the moving distance of the object lens and the difference of two values of the driving voltage can be expressed as a displacement gain, wherein the units of the displacement gain is distance/voltage. The displacement gain can also be thought of as a DC sensitivity of the focusing actuator.

Take the optical disc identification procedure as an example. By measuring the time for the focal point of the laser light generated by one of the LDs to move between a surface layer and a reflective layer of the optical disc 120, or by measuring the voltage change of the driving voltage, the distance between the surface layer and the reflective layer can be determined according to the measuring result and a predetermined displacement gain of the focusing actuator. Roughly speaking, an optical disc with shorter distance between the surface layer and the reflective layer will be identified as a DVD type optical disc. An optical disc with longer distance between the surface layer and the reflective layer will be identified as a CD type optical disc.

Optical storage devices of the prior art use a fixed value as the predetermined displacement gain of the focusing actuator, and use the fixed displacement gain as a parameter to process procedures such as the layer jump procedure or the optical disc identification procedure. However, because there are always slight differences between each focusing actuator, it is almost impossible to manufacture optical storage devices having a common and exact value on the displacement gain. Hence, it's apparent that using a common and fixed value as the displacement gain for each optical storage device is not the best solution.

SUMMARY

It is therefore an objective of the present invention to provide a method for determining a relationship between difference of object lens location and a driving control effort in an optical storage device.

According to one embodiment of the present invention, a method used in an optical storage device is disclosed. The optical storage device includes a pickup head. The pickup head includes an object lens and a light source. The object lens focuses laser light generated by the light source onto a focal point. A driving control effort is used to change the location of the object lens. The method is used to determine a relationship between the difference of object lens location and the driving control effort, and comprises the following steps: (a) providing the driving control effort to move the object lens; (b) measuring a first parameter when a signal reaches a first state; (c) measuring a second parameter when the signal reaches a second state; and (d) determining the relationship between the difference of object lens location and the driving control effort according to a predetermined distance, the first parameter, and the second parameter.

It is an advantage of the present invention that an unique value representing the relationship between the difference of object lens location and the driving control effort could be determined accurately. The measured relationship could be used to calibrate the optical storage device; hence the controlling accuracy of the optical storage device is enhanced.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional optical storage device.

FIG. 2 is a flow chart according to a first embodiment of the present invention.

FIG. 3 is a diagram showing how the driving voltage and the focusing error signal, which is briefly called as FE signal, changes while the flowchart of FIG. 2 is being processed.

FIG. 4 is a flow chart according to a second embodiment of the present invention.

FIG. 5 is a diagram showing how the driving voltage and the FE signal change while the flowchart of FIG. 4 is being processed.

FIG. 6 is a block diagram of an optical storage device according to the present invention.

DETAILED DESCRIPTION

An idea of the present invention is to measure the displacement gain (i.e. a relationship between the difference of object lens location and a driving control effort) by using a specific distance as a reference basis. In the following description, the method disclosed by the present invention is used in an optical storage device containing a CD laser diode (which is a first light source of the optical storage device), a DVD laser diode (which is a second light source of the optical storage device), and an object lens. The object lens is used to focus the laser light generated by the CD laser diode onto a first focal point or focus the laser light generated by the DVD laser diode onto a second focal point. A driving control effort is used to control the location of the object lens (in this example, the driving control effort is a driving voltage, however, the driving control effort could also be a driving current or other kind of control means).

With the progression of lens manufacturing technology, the focal length of an object lens corresponding to the CD laser light, i.e., the distance between the first focal point and the object lens, can have a highly accurate value. The focal length of the object lens corresponding to the DVD laser light, i.e., the distance between the second focal point and the object lens, can also have a highly accurate value. Hence, the distance between the first focal point and the second focal point, which is hereafter referred to a first distance WD_delta, which can be derived from the characteristics of pickup head, is also a substantially fixed value. In the first embodiment of the present invention, the first distance WD_delta is used as the abovementioned specific distance.

FIG. 2 shows a flow chart according to a first embodiment of the present invention. The steps are described in more detail in the following.

-   Step 200: Turn on the CD laser diode. Change the driving voltage to     move the object lens down until it reaches the lowest end. In this     step, “move down” means to move the object lens away from the     surface of the optical disc. -   Step 210: Continue to change the driving voltage to keep moving the     object lens up. In this step, “move up” means to move the object     lens toward the surface of the optical disc. -   Step 220: Determine whether the first focal point has passed through     a first layer of the optical disc by inspecting the characteristics     of a focus error signal FE. Measure the value of the driving voltage     as a first voltage value V1 when the first focal point passes     through the first layer of the optical disc. In this embodiment,     when an s-curve is detected in the FE signal, it is considered that     the first focal point has passed through the first layer of the     optical disc. However, other servo control signals can also be used     as basis to determine whether the first focal point has passed     through the first layer of the optical disc. In addition, the first     layer mentioned here could be a surface layer or a reflective layer. -   Step 230: Turn off the CD laser diode, and then turn on the DVD     laser diode. Determine whether the second focal point has passed     through the first layer of the optical disc by inspecting the     characteristics of the FE signal. Measure the value of the driving     voltage as a second voltage value V2 when the second focal point     passes through the first layer of the optical disc. -   Step 240: Determine the displacement gain D_gain according to the     V1, V2, and WD_delta. Here the displacement gain D_gain is the     relationship between the displacement of the object lens     corresponding to the change of the driving voltage.

Clearly, since the location of the first layer remains unchanged during the above-mentioned steps, the moving distance of the object lens between the step 220 and 230 will substantially be equal to the first distance WD_delta. With the first distance WD_delta used as a basis, the value of the displacement gain D_gain can then be determined. Please refer to FIG. 3, FIG. 3 is a diagram showing how the driving voltage and the FE signal change while the flowchart of FIG. 2 is being processed.

If the first layer is a surface layer of the optical disc, the value of the displacement gain D_gain can be determined according to the following equation (1). If the first layer is a reflective layer of the optical disc, the value of the displacement gain D_gain can be determined according to the following equation (2).

D_gain=WD_delta/|V2−V1|  Equation 1

D_gain=WD_delta/[|V2−V1|×n]  Equation 2

Please note that in the above equations it is assumed that a CD disc or a DVD disc has a common refractive index n for CD laser light and DVD laser light. Normally the value of n can be assumed to be 1.55. For convenience, the above-mentioned assumption will also be used in the following description. When a CD disc or a DVD disc has different refractive indices for CD laser light and DVD laser light, the equation 2 will not be correct. However, a more complex equation can be derived by a person of ordinary skill in the art with the working distance and focal distance of each laser light. The refractive index n can also be determined using experimental statistics. Hence the method provided by the present invention remains practicable.

Additionally, in the steps 220 and 230, it is also possible to measure a first time T1 when the first focal point passes through the first layer of the optical disc instead of measuring V1, and to measure a second time T2 when the second focal point passes through the first layer of the optical disc instead of measuring V2. The change of the driving voltage can be determined according to the time difference (T2−T1). When the change rate of the driving voltage with respect to time has a fixed slope, the deduction process becomes quite simple. The following paragraph is focused on the situation when the slope has a fixed value C.

If the first layer is a surface layer of the optical disc, the value of the displacement gain D_gain can be determined according to the following equation (3). If the first layer is a reflective layer of the optical disc, the value of the displacement gain D_gain can be determined according to the following equation (4). The value n in equation (4) is the reflective index inside the optical disc.

D_gain=WD_delta/[|T2−T1|×C]  Equation 3

D_gain=WD_delta/[|T2−T1|×C×n]  Equation 4

The abovementioned first embodiment of the present invention can be used in an optical storage device having at least two laser diodes. Alternatively, the second embodiment of the present invention can be used in an optical storage device having at least one laser diode. The optical storage device implementing the second embodiment of the present invention contains a pickup head. The pickup head includes an object lens and a laser diode, which can be a CD laser diode or a DVD laser diode. The object lens focuses the laser light generated by the laser diode onto a focal point. A driving voltage (it could also be a driving current or other kind of control means) is used to control the movement of the object lens hence the location of the object lens can be controlled.

In this second embodiment, the specific distance is a distance that the s-curve of the FE signal has a linear region when the focal point passes through a first layer of the optical disc. The abovementioned first layer could be a surface layer or a reflective layer.

FIG. 4 shows a flow chart according to the second embodiment of the present invention. The steps are described in more detail in the following.

-   Step 400: Turn on the laser diode. Change the driving voltage to     move the object lens down until it reaches the lowest end. In this     step, “move down” means to move the object lens away from the     surface of the optical disc. -   Step 410: Continue to change the driving voltage to keep moving the     object lens up. In this step, “move up” means to move the object     lens toward the surface of the optical disc. -   Step 420: Inspect the characteristics of the FE signal. After the FE     signal passes through a predetermined threshold, measure the value     of the driving voltage as a first voltage value V1 when the FE     signal reaches a maximum value MAX. Using the predetermined     threshold prevents an erroneous decision caused by noise from     occurring. Additionally, other servo control signals could also be     used here instead of the FE signal. -   Step 430: Continue to inspect the characteristics of the FE signal.     Measure the value of the driving voltage as a second voltage value     V2 when the FE signal reaches a minimum value MIN. The region that     the FE signal changes from MAX to MIN can be viewed as the linear     region. -   Step 440: Determine a displacement gain D_gain according to the     values V1, V2, and a value Def_delta. Here the displacement gain     D_gain is the relationship between the displacement of the object     lens corresponding to the change of the driving voltage. The     Def_delta is the distance that the object lens must move to allow     the FE signal to pass through its linear region, and the value of     Def_delta can determined through experimental statistics.

Please refer to FIG. 5. FIG. 5 is a diagram showing how the driving voltage and the FE signal change while the flowchart of FIG. 4 is being processed.

It is also possible to use the central part of the linear region instead of the whole linear region. The central part of the linear region refers to the region where the FE signal changes from (MAX×A) to become (MAX×B), wherein both A and B are smaller than ±1; or from (MAX+C) to become (MAX+D). As a design choice, the designer can also use the whole s-curve instead of only the linear region of the s-curve. However, by using only the linear region, higher accuracy can be achieved.

If the first layer is a surface layer of the optical disc, the value of the displacement gain D_gain is determined according to the following equation (5). If the first layer is a reflective layer of the optical disc, the value of the displacement gain D_gain is determined according to the following equation (6). The value n in equation (6) is the reflective index inside the optical disc.

D_gain=Def_delta/|V2−V1|  Equation 5

D_gain=Def_delta/[|V2−V1|×n]  Equation 6

Additionally, in the steps 420 and 430, it is also possible to measure a first time T1 when the FE signal reaches the value MAX instead of measuring V1, and to measure a second time T2 when the FE signal reaches the value MIN instead of measuring V2. The change of the driving voltage can be determined according to the time difference (T2−T1). When the change rate of the driving voltage with respect to time has a fixed slope, the deduction process will become quite simple. The following paragraph is focused on the situation when the slope has a fixed value C.

If the first layer is a surface layer of the optical disc, the value of the displacement gain D_gain is determined according to the following equation (7). If the first layer is a reflective layer of the optical disc, the value of the displacement gain D_gain is determined according to the following equation (8). The value n in equation (8) is the reflective index inside the optical disc.

D_gain=Def_delta/[|T2−T1|×C]  Equation 7

D_gain=Def_delta/[|T2−T1|×C×n]  Equation 8

By using the method provided by the present to determine the exact value of the displacement gain of the focusing actuator of a particular optical storage device, the optical storage device can be calibrated. The optical disc identification procedure or layer jump procedure would be controlled more accurately. In addition, the determined displacement gain can also be used by the controller(s) of the optical storage device to process frequency response calibration.

Please note that the abovementioned moving direction in each step can also be reversed, that is, to first move the object lens up and then move it down.

FIG. 6 shows an optical storage device 600 according to the present invention, which is capable of measuring a displacement gain through the abovementioned method. The optical storage device 600 contains a spindle motor 610, an optical disc 620, a pickup head 630, a signal processing unit 640, a focusing controller 650, a micro controller unit (MCU) 660, a power driver 670, and an amplifier unit 680. The pickup head 630 includes an object lens and a focusing actuator. The focusing actuator controls the location of the object lens according to a driving voltage generated by the power driver 670. Different from that of the prior art, the micro controller unit 660 of the optical storage device 600 includes a determining unit 665, which determines the displacement gain of the focusing actuator by using the method provided by the present invention. In addition, the micro controller unit 660 can also use the displacement gain determined by the determining unit 665 to calibrate the optical storage device 600.

Disregarding the amplifier unit 680, after the displacement gain is determined, each component of the closed loop formed by the pickup head 630, the signal processing unit 640, the focusing actuator 650 and the power driver 670 can be calibrated. However, in this embodiment there is an amplifier unit 680 located between the focusing controller 650 and the power driver 680. To allow the micro controller unit 660 to adjust the amplifier unit 680 directly, it is easier to implement the system architecture shown in FIG. 6. Additionally, although the determining unit 665 is located in the micro controller unit 660, it is also possible to design the determining unit 665 as an isolated functional block.

In contrast to the prior art, the present invention measures the displacement gain of the focusing actuator of each unique optical storage device. After the displacement gain is determined, the optical storage device can be calibrated properly, hence corresponding control procedure becomes more accurate. In addition, the optical storage device according to the present invention comprises a determining unit, which determines a displacement gain for the optical storage device and the displacement gain can be used to calibrate the optical storage device. Hence related control procedures in the optical storage device are more accurate than that of the prior art.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

1. A method used in an optical storage device comprising a pickup head, wherein the pickup head comprises an object lens and a light source, the object lens focuses laser light generated by the light source onto a focal point, a driving control effort is used to change the location of the object lens, and the method is used to determine a relationship between the difference of object lens location and the driving control effort, the method comprising: (a) providing the driving control effort to move the object lens; (b) measuring a first parameter when a signal reaches a first state; (c) measuring a second parameter when the signal reaches a second state; and (d) determining the relationship between the difference of object lens location and the driving control effort according to a predetermined distance, the first parameter, and the second parameter.
 2. The method of claim 1, wherein in step (b), the signal reaches the first state means that the signal reaches a first specific value after passes through a predetermined threshold; and in step (c), the signal reaches the second state means that the signal reaches a second specific value after passes through the first specific value.
 3. The method of claim 2, wherein the first specific value is substantially a maximum value of the signal, and the second specific value is substantially a minimum value of the signal.
 4. The method of claim 2, wherein the first specific value is substantially a maximum value of the signal mathematically operated by a first factor, and the second specific value is substantially a minimum value of the signal mathematically operated by a second factor.
 5. The method of claim 2, wherein the first parameter is a first driving control effort value of the driving control effort when the signal reaches the first state; and the second parameter is a second driving control effort value of the driving control effort when the signal reaches the second state.
 6. The method of claim 5, wherein between step (b) and step (c) the focal point passes through a surface layer of an optical disc; and in step (d): the relationship between the difference of object lens location and the driving control effort is determined according to a function of the predetermined distance, a driving control effort difference; wherein the driving control effort difference is a difference value between the second driving control effort value and the first driving control effort value.
 7. The method of claim 5, wherein between step (b) and step (c) the focal point passes through a reflective layer of an optical disc; and in step (d): the relationship between the difference of object lens location and the driving control effort is determined according to a function of the predetermined distance, a driving control effort difference and a reflective index; wherein the driving control effort difference is a difference value between the second driving control effort value and the first driving control effort value.
 8. The method of claim 2, wherein the first parameter is a first time when the signal reaches the first state; the second parameter is a second time when the signal reaches the second state, and in step (a) the change rate of the driving control effort with respect to time is a first slope.
 9. The method of claim 8, wherein between step (b) and step (c) the focal point passes through a surface layer of an optical disc; and in step (d): the relationship between the difference of object lens location and the driving control effort is determined according to a function of the predetermined distance, a time difference and the first slope; wherein the time difference is a difference value between the second time and the first time.
 10. The method of claim 8, wherein between step (b) and step (c) the focal point passes through a reflective layer of an optical disc; and in step (d): the relationship between the difference of object lens location and the driving control effort is determined according to a function of the predetermined distance, a time difference and a reflective index; wherein the time difference is a difference value between the second time and the first time.
 11. The method of claim 1, wherein the signal is a servo control signal.
 12. The method of claim 1, wherein the predetermined distance is determined through experimental statistics.
 13. The method of claim 1, further comprising calibrating the optical storage device in accordance with the determined relationship between the difference of object lens location and the driving control effort.
 14. The method of claim 1, further comprising processing frequency response calibration in accordance with the determined relationship between the difference of object lens location and the driving control effort. 